The present invention relates to furnaces for melting metals.
In the melting of non-ferrous metals, such as aluminum alloys, fossil fuels (liquified gas, natural gas, and oil) are commonly utilized. The fossil fuels are mixed with air and ignited, producing a hot combustion flame and combustion gases. These products of combustion (i.e., the flame and gases), are utilized in different ways, depending upon the type of furnace, to produce the desired result of melting or breakdown of the metal charge, and then superheating to a temperature to be held for pouring.
Although many variations exist, there are two broad types of fossil fuel fired, non-ferrous metal, melting furnaces, namely, crucible and reverberatory. Each type utilizes the products of combustion in a different manner, with associated advantages and disadvantages.
Crucible furnaces are typically comprised of a cylindrical insulating/refractory wall and a crucible or pot. The insulating/refractory wall is in spaced relationship to the crucible to provide an annular heating chamber for circulating the flame and products of combustion. The most efficient furnaces utilize an enclosed barrier/furnace interface so that the pressurized products of combustion cannot blow back out of the annular combustion space. Heating of the crucible is accomplished by the scrubbing of the flame and combustion gases along the periphery of the crucible as they travel their path to the exhaust port. The flame is preferably directed tangentially to the crucible to avoid hot spots which could shorten the life of the crucible. A vertical stack is commonly connected to the exhaust port in the insulating wall to exhaust combustion flame and gases away from the furnace operator.
One major disadvantage of these crucible furnaces is that they require a relatively long period of time to transform metal from a solid to a molten state. The relatively long melt time associated with crucible furnaces is due, in part, to the fact that the heat for melting the charge must be conducted through the crucible wall. Also, the high temperatures involved in crucible melting, as well as contamination problems associated with, e.g., ferrous, crucible materials, dictate that the crucible be constructed of refractory materials, such as silicon carbide or graphite. Such refractory materials are inherently poor conductors of heat, and thus, a substantial period of time is required to conduct sufficient heat to the interior of the crucible to melt the charge. The poor heat transfer characteristics of refractory materials make crucible furnaces extremely inefficient, and thus, tremendous amounts of energy are required for their operation. For example, one pound of aluminum requires approximately 511 BTU of energy to convert it to the molten state and then superheat it to 1400.degree. F. (a typical pouring temperature). A typical crucible furnace requires 3,000 to 7,000 BTU to melt one pound of aluminum, thus yielding an efficiency of only about 7 to 17%.
A further disadvantage of crucible furnaces is that the crucibles are heated to extremely high temperatures from their exterior side, thereby creating high temperature gradients within the walls of the crucibles. These temperature gradients tend to cause the crucibles to fracture, typically after only 3 to 6 months of operation. Replacement of the crucibles involves significant expense, and this adds substantially to the cost of foundry operations.
Although the crucible furnace has many disadvantages, one major advantage is flexibility. A change of alloy can be accomplished readily by emptying the crucible, scraping down the excess material from the sidewalls of the crucible, disposing of this material, and then recharging with a new alloy. This takes only a matter of minutes, and can be accomplished, for example, after every shift, if necessary. A further advantage of crucible furnaces is that they experience relatively low metal losses due to oxidation. Typically, such metal losses amount to about 3 to 5%.
In contrast to crucible furnaces, reverberatory furnaces are usually square or rectangular in shape and are comprised of a refractory hearth for holding the metal charge. The hearth is surrounded by a roof and walls having a first layer of insulating material, adjacent to the outside of the furnace, and a second layer of refractory material adjacent to the combustion zone. The burners are placed in the roof or top of the furnaces, aimed at the hearth, and separated from the hearth by a predetermined air space. The burners are also pressurized and of two distinct types: (a) luminous flame burners, or those whose flame and products of combustion tend to impinge on the surface of the metal charge and (b) radiant burners, or those that have no contact between the flame and combustion gases, and rely solely on the transfer of radiant energy from a heated ceramic shield to the charge.
Two distinct types of reverberatory furnaces are in use at the present time, (a) dry hearth, and (b) wet hearth, or wet bath. The dry hearth furnace consists of a sloped hearth on which the metal charge is placed. Upon melting, the molten metal runs down the hearth into a secondary holding chamber, that requires auxillary heating. The wet hearth has a holding chamber, having a refractory hearth upon which melting and holding takes place.
The combustion of fossil fuels result in copious quantities of hydrogen and oxygen. Hydrogen is readily absorbed by molten aluminum and is released during solidification, resulting in porosity or voids. Molten aluminum rapidly oxidizes in the presence of oxygen. Consequently, molten metal produced in a typical reverberatory furnace is usually poor in quality, yielding relatively inferior ingots and/or castings. Moreover, the oxidation results in a relatively large metal loss, typically in the range of 5 to 12% of the charged metal.
In contrast to crucible furnaces, reverberatory furnaces are relatively sufficient. For example, a typical reverberatory furnace may utilize about 1,500 to 4,000 BTU to melt one pound of a aluminum, yielding an efficiency of 12.8 to 34%. However, reverberatory furnaces do not have the flexibility of crucible furnaces, and a change of alloy can be an involved, time consuming, and costly endeavor.
Thus, crucible and reverberatory furnaces each have advantages over the other. The crucible furnace is more flexible in use, and produces a cleaner molten metal. The reverberatory furnace, on the other hand, out-performs the crucible furnace immensely in speed of melting, since heat does not have to be transferred through a wall of refractory material. While both types of furnaces have their devoted users, there exists a need for a single furnace which combines the best features of both furnace types.